FLATLINED: St. Vincent's officials allegedly ran the now-defunct hospital's finances into the ground in order win approval to sell it off. Photo: NY Post: G.N. Miller

It was an unholy vow of poverty.

The Catholic-charity-run St. Vincent’s Hospital is under investigation by the Manhattan district attorney’s fraud unit, which is probing whether honchos purposely tanked its finances so it could be sold to a private developer, The Post has learned.

Tens of millions of taxpayer dollars went up in smoke during the center’s collapse, and DA investigators are examining sky-high payments to top executives and private consultants at the hospital founded by the Sisters of Charity in 1849, said multiple sources familiar with the probe.

Former CEO Henry Amoroso, once a top church official in the Newark Archdiocese, oversaw the runaway spending, which led to $1 billion in debt and bankruptcy before the hospital closed in April 2010. Sisters of Charity President Jane Iannucelli serves as vice chairwoman of the St. Vincent board.

Going broke allowed the hospital to get an OK from the state Health Department to sell to the Rudin family, which is building luxury housing on the site. Without bankruptcy, state officials would not have been permitted the hospital to shut down, the sources said.

“This was a well-thought-out plan,” said Tom Shanahan, a lawyer for a group of former St. Vincent doctors and nurses suing St. Vincent’s. “They wanted out and had to justify it to the state. They were running it into the ground.”

DA Cy Vance’s team is looking into whether vendors double-billed for services, gave kickbacks for contracts and hired relatives of hospital employees, sources said.

Two senior administrators each got more than $1 million after leaving their jobs — and continued to be listed as employees on tax returns.

Virginia Sweeney, the top nursing officer whose employment ended in 2006, got paid $1.3 million between 2007 and 2009. Jane Connorton, the president who left in 2004, pocketed $1 million over the next four years.

Amoroso and Iannucelli did not return calls for comment.

A spokeswoman for the hospital said it had not been contacted by the DA’s Office. “We are not aware of any investigation,” said Brenda Adrian.